A bolted assembly comprises first and second parts having at least one facing hole, a screw passing through said holes and a nut screwed onto the screw. Such assemblies are initially tightened to a predetermined torque value. However, under normal operating conditions, the bolted assemblies can be subject to both thermal stresses and mechanical stresses, like vibrations, which can lead to modifications to the tension initially introduced into the bolts during the initial tightening operation. In turn, these variations can lead to a partial or even total loss of the function imparted to the bolted assembly.
This is, for example, the case in dismantleable heat exchangers with bolted assemblies, when they are subject to high thermal transients. Thus, these transients can give rise to overtightening during hot shocks. Such overtightening may lead to permanent deformation of the bolts or studs or to deformation of the assembled parts. This can lead to a sealing loss in the assembly, when the exchanger returns to normal temperature conditions. Thermal transients can also give rise to cold shocks, which produce undertightening effects which, although not leading to a deterioration in the structures, particularly the bolts, reduce the crushing or compression pressures of the joints and also cause leaks.
The prior art discloses various measures for reducing the extent of the stress variations on bolted assemblies in use.
In the case of existing equipment and when the initial tightening stresses and variations are small, it is known to place beneath the nuts or bolt heads helical washers called spring or lock washers. Such washers are mass produced from a ring of small height with a trapezoidal section, followed by slitting a deformation in order to form the equivalent of a spring with one turn. In the free state the height of such a washer is approximately twice the height of the ring, which corresponds to a helical pitch close to the thickness. When the washer is compressed, it has the flat shape of a conventional washer. All the initial deflection or sag is cancelled out by the tightening force.
The function of these helical washers is twofold, in that they act as a nut lock preventing rotation thereof and can also compensate by spring effect for a slight reduction in the tension in the bolts. However, it is pointed out that these washers only act in the case of a pressure reduction. In the opposite case of a pressure increase, they permit no compensation of the tension in the bolts because, as stated hereinbefore, all the initial deflection or sag is cancelled out, and consequently such spring washers are then shaped like a conventional washer.
The very way in which these components are manufactured implies that the forces which the washers have to transmit in their elastic operating range are lower than the force necessary for the initial deformation of the ring. Thus, bearing in mind the small dimensions of said ring, the forces which can be transmitted in operation are also small.
The prior art also discloses conical washers, called cupped washers. In their useful zone, these washers can transmit higher forces than in the case of spring washers, but they are still not very high. If the transmitted force is to be increased, it is necessary to superimpose several such washers. However, the behavior of this assembly becomes non-linear and variable as a function of the evolution direction of the charge or load. It is in particular highly dependent on the contact conditions between the washers. Moreover, there is a risk of deterioration of these washers if, as a result of incidents, the sag imposed exceeds approximately two-thirds of the washer height. Furthermore, in the case of a high tightening force, the stresses in operation in the washer exceed those permitted by the nuts, studs or bolts of pressure equipment, such as a steam generator or heat exchanger.
In the case of such highly stressed equipment or any other equipment where it is not possible to satisfy the above conditions, it is still possible to lengthen the bolts in order to increase the elastic deformation range of their shanks. This means that a spacer has to be introduced round the bolts to ensure the continuity of the assembly. However, to be effective, this measure requires a significant lengthening of the bolts, which is often incompatible with the assembly or overall dimension conditions.